Weed and grass trimmers have been developed which employ a rotatable hub with a short length of flexible nylon or other plastic line extending from the hub. When the hub is rotated, the tip of the line extending from the hub provides the cutting or trimming action. Grass and weed trimmers using this principle of operation have become popular for their versatility of use and because the flexible trim line is safer to use than rigid, rotating steel blades.
Various types of devices have been developed for using such trim lines. Typically, rotating line trimmers or string trimmers employ a line which generally has a circular cross section. The line, in many trimmers, is wound on a storage reel in the hub of the device, and is played out of a hole in the hub in discrete amounts as the end breaks off or wears off. In operation, the section of the line which extends outwardly from the rotating head attains a substantially straight configuration as a result of the centrifugal force. As the line is swung in a circle by the rotating head, it cuts grass, vines, brush and the like, with which it comes into contact.
The circular cross-sectional configuration of trimmer lines generally leaves a ragged edge on the ends of the blades of grass or plants cut with such trimmers. This is a result of the relatively blunt cutting edge, which strikes the grass and breaks it off. As a result, the ragged grass tends to turn brown shortly after cutting, resulting in a less desirable appearance than if the grass is cut or broken off cleanly along a straight line.
The U.S. patent to Mize U.S. Pat. No. 4,186,239 discloses a modification of the normal cross section monofilament line used in weed and grass trimmers. In the Mize device, periodic irregular cross-sectional variations are formed along the length of the line for the purpose of reducing fibrillation of the filament. This patent also discloses line extrusions of generally triangular or star shapes, in which the cutting edges are provided with notches or irregularities in planes perpendicular to the central axis of the cutting line. In all of the variations which are shown in this patent, however, a relatively large blunt edge contacts the plant to effect the cutting.
Several U.S. patents are directed to rotary line cutters, in which the cutting lines have triangular cross sections. Three such patents are Jacyno U.S. Pat. No. 4,118,865; Jones et al. U.S. Pat. No. 4,905,465; and Jones et al. U.S. Pat. No. 5,048,278. The lines of all of these patents are of a relatively short finite length. They are not extended from a supply reel as they wear out. Theoretically, however, the triangular cross sections are intended to present a sharp cutting edge to the grass or the plants to be cut. In actual practice, however, if the orientation of the triangle causes one of the flat triangle surfaces to be presented to the plant in the plane of impact, a flat, blunt surface effects the cut, instead of one of the sharp triangular edges.
A different modification of the conventional circular cross-sectional trimmer line is disclosed in the United States patent to Mickelson U.S. Pat. No. 4,869,055. The line disclosed in this patent has a generally star-like cross-sectional configuration, in which longitudinally extending sharp edges are separated by depressed areas (concave or sharp angular configurations) between any two adjacent edges. The depressed areas are significant (at least 10 percent of the length of a straight line from the tips of adjacent edges); so that the sharp edges always are at the outermost portion of the line. The intent of the line disclosed in this patent is that, irrespective of the orientation of the line, a sharp edge always strikes the plant or weed first. The line is symmetrical in its cross section; and as it rotates, sharp edges, by design, always strike the plant first. Since the tips of the sharp edges define the external diameter of the line which must be fed through the opening in the head of the trimmer, the depressed areas cause the line to have considerably less mass than a corresponding standard, circular, cross-sectional line of the same nominal diameter. In fact, the mass of a line manufactured in accordance with the disclosure of the Mickelson '055 patent has only approximately two thirds of the mass of a standard, circular cross-sectional line. Because the sharp edges of the Mickelson line always strike the plant first, these edges quickly wear away; and the reduced mass of the line also causes this line rapidly to wear out. In addition, the reduced amount of material causes a reduction in the mass, and therefore the force at which the line strikes vegetation, causing the line to be of significantly reduced efficiency when plants other than grass are encountered by the line.
Another patent which discloses a line with a somewhat star-shaped cross section is the U.S. patent to Bair U.S. Pat. No. 4,047,299. The cutting line described in this patent is indicated as having a star-shaped cross section. The line is described as being "non circular or non-round" with longitudinal ribs to form this cross-sectional configuration. The Bair patent, however, is silent as to the relative dimensions of the ribs and the rest of the line, and merely states that the longitudinal ribs are included to provide additional stiffness and strength, facilitating the cutting of heavier vegetation.
The diameter of lines used for string trimmers typically is in a range of 0.040 inches to 0.155 inches. The larger diameters typically are used for commercial trimmers designed for trimming relatively heavy vegetation. The larger cross-sectional sizes of the cutting lines require greater power to operate the trimmer. Consequently, non-commercial trimmers typically use lines of the smaller diameters (for example, 0.040 inches) with a smaller, lighter weight motor.
It is desirable to provide a line for rotating string trimmers which overcomes the disadvantages of the prior art devices mentioned above, and which provides relatively clean, sharp cuts in grass and also is capable of use in heavy vegetation without excessive wear.